First I was having some errors to play .webm videos with the following message: "no video with supported format and MIME type found"
Then I found somewhere a guy that solved this problem using
sudo ...

Audio within mp4 and flv files is AAC, and a proper container for that is m4a. So, in order to extract sound from mp4 and flv I use a command like
gnome-terminal -e "parallel avconv -i '{}' -map 0:1 ...

I take photos and videos using Cheese on Ubuntu 12.04. They are in WebM formats.
How to convert them in other more conventional formats for making them compatible with other OS and devices (Windows, ...

Sorry to disturb you but I'm in a kind of a trouble.
I have to convert 76 mp4 files to webm for the purpose of a website that uses HTML5 videos. I'm talking about 10 Gb of mp4 files...
I know I can ...